Sorry for the slow replies - I am about to move house and have been packing and sorting out years worth of crap.

Dorian is a Sector made from hundreds of clusters. Each cluster has hundreds of thousands of stars. In crowded clusters, there are hundreds of inhabited systems. When the Droids invaded Dorian, there were millions of inhabited systems.

There is a mis-print here, the human race was decimated down to 0.02% of its previous grandure. There are still trillions of people left. Then Dorian was invaded but evacuations had become inevitably slick.

What stopped the Droids is that a virus was inserted into their programming that questioned simply "What is life?". This turned the Droids against each other and meant only those Droid producing systems close to the human race attack it. This is a tiny number compared to the mass of Droids.